Tetrasomy 18p: The challenges of noninvasive prenatal testing and combined test.

Circulating cell-free DNA, having no procedural risk of miscarriage, is the most suitable sample for a noninvasive prenatal testing (NIPT). Here we report on a boy, who came to our attention for hypotonia and psychomotor delay when he was 16 months old. During the pregnancy his mother performed a NIPT that resulted compatible with the presence of trisomy 18. A confirmatory invasive procedure has been proposed, but not performed, because the family preferred to follow a conservative line. A series of ultrasound excluded the presence of any major malformations. Genetic tests, performed after birth, revealed the presence of a tetrasomy 18p with an extra isochromosome 18p. Our report wants to underline the importance of an invasive procedure and consequent cytogenetic analysis in case of NIPT results indicating autosomal trisomies because under it may hide the presence of other structural chromosomal anomalies. These conditions are normally not visible at prenatal ultrasound or using combined test (CT) and the only identifiable not invasive sign could be the results of high risk at NIPT.